This application relates generally to combustors and, more particularly, to gas turbine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Most aircraft engines are able to meet current emission standards using combustor technologies and theories proven over the past 50 years of engine development. However, with the advent of greater environmental concern worldwide, there is no guarantee that future emissions standards will be within the capability of current combustor technologies.
In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures which do not allow the fuel-air reaction to proceed to completion (HC and CO). A small window exists where both pollutants are minimized. For this window to be effective, however, the reactants must be well mixed, so that burning occurs evenly across the mixture without hot spots, where NOx is produced, or cold spots, when CO and HC are produced. Hot spots are produced where the mixture of fuel and air is near a specific ratio when all fuel and air react (i.e. no unburned fuel or air is present in the products). This mixture is called stoichiometric. Cold spots can occur if either excess air is present (called lean combustion), or if excess fuel is present (called rich combustion).
Known gas turbine combustors include mixers which mix high velocity air with a fine fuel spray. These mixers usually consist of a single fuel injector located at a center of a swirler for swirling the incoming air to enhance flame stabilization and mixing. Both the fuel injector and mixer are located on a combustor dome.
In general, the fuel to air ratio in the mixer is rich. Since the overall combustor fuel-air ratio of gas turbine combustors is lean, additional air is added through discrete dilution holes prior to exiting the combustor. Poor mixing and hot spots can occur both at the dome, where the injected fuel must vaporize and mix prior to burning, and in the vicinity of the dilution holes, where air is added to the rich dome mixture.
Properly designed, rich dome combustors are very stable devices with wide flammability limits and can produce low HC and CO emissions, and acceptable NOx emissions. However, a fundamental limitation on rich dome combustors exists, since the rich dome mixture must pass through stoichiometric or maximum NOx producing regions prior to exiting the combustor. This is particularly important because as the operating pressure ratio (OPR) of modem gas turbines increases for improved cycle efficiencies and compactness, combustor inlet temperatures and pressures increase the rate of NOx production dramatically. As emission standards become more stringent and OPR""s increase, it appears unlikely that traditional rich dome combustors will be able to meet the challenge.
One state-of-the-art lean dome combustor is referred to as a trapped vortex combustor because it includes a trapped vortex incorporated into a combustor liner. Such combustors include a dome inlet module and an elaborate fuel delivery system. The fuel delivery system includes a spray bar that supplies fuel to the trapped vortex cavity and to the dome inlet module. The spray bar includes a heat shield that minimizes heat transfer from the combustor to the spray bar. Because of the velocity of air flowing through the combustor, recirculation zones may form downstream from the heat shield and the fuel and air may not mix thoroughly prior to ignition. As a result of the fuel being recirculated, a flame may damage the heat shield, or fuel may penetrate into the heat shield and be auto-ignited.
In an exemplary embodiment, a combustor for a gas turbine engine operates with high combustion efficiency and low carbon monoxide, nitrous oxide, and smoke emissions during engine power operations. The combustor includes at least one trapped vortex cavity, a fuel delivery system that includes at least two fuel circuits, and a fuel spray bar assembly that supplies fuel to the combustor. The two fuel stages include a pilot fuel circuit that supplies fuel to the trapped vortex cavity and a main fuel circuit that supplies fuel to the combustor. The fuel spray bar assembly includes a spray bar and a heat shield. The spray bar is sized to fit within the heat shield and includes a plurality of injector tips. The heat shield includes aerodynamically-shaped upstream and downstream sides and a plurality of openings in flow communication with the spray bar injection tips.
During operation, fuel is supplied to the combustor through the spray bar assembly. Combustion gases generated within the trapped vortex cavity swirl and stabilize the mixture prior to the mixture entering a combustion chamber. The heat shield improves fuel and air mixing while preventing recirculation zones from forming downstream from the heat shield. During operation, high heat transfer loads develop resulting from convection due to a velocity of heated inlet air and radiation from combustion gases generated within the combustor. The heat shield protects the spray bar assembly from heat transfer loads. Furthermore, the spray bar assembly prevents fuel from auto-igniting within the heat shield. Because the fuel and air are mixed more thoroughly, peak flame temperatures within the combustion chamber are reduced and nitrous oxide emissions generated within the combustor are also reduced. As a result, a combustor is provided which operates with a high combustion efficiency while controlling and maintaining emissions during engine operations.